I'm trying to figure out the best way to heat the second floor of my house. Its currently heated by a space heater but I want to get some kind of central heating system set up, but I'm not sure what set up makes the most sense for me.

The first floor of my house has forced air heat from a basement furnace, but the ductwork doesnt extend up to the second floor. I am planning on renting the second floor out to one of my friends. In light of this, all the subcontractors I've talked to have been advising me to just install a second furnace for the second floor. However, this doesnt seem to make a ton of sense to me. In terms of resale value, who is going to want to buy a house with two furnaces? Also, if I decide after a while that I dont want a roommate anymore then I am going to be stuck paying two furnace bills.

Couldnt I just use my existing furnace and extend the ductwork up to the second floor? Is that going to be more expensive than getting a second furnace? I want to get central air and upgrade my existing furnace eventually anyway. Would it make more sense to install a new furnace in the basement + central air, and extend the ductwork up to the second floor?

While I see your point, consider the fact that the renter may want it warm while you want it cool and vice-versa.
Yes a second furnace will be more than a ductwork extension.
You also have to think about separate utility meters if you don't want to include utilities in the rental amount.

What size is the space heater? This will tell you how many BTU/hr you need to deliver to that area.

HVAC design is more difficult than electronic circuit design, but if you're up to it have your local library borrow Manual D and Manual J through interlibrary loan. The lookup tables solve the equations for you.

You also need
http://www.google.com/search?client=safari&rls=en&q=%22outside+design+temperature%22+chicago&ie=UTF-8&oe=UTF-8
The Stack Effect is in your favor for heating, on this one. Cooling is another story.

I lived in a duplex with one furnace and one thermostat. It sucked if the thermostat was on the other side of the wall. I'd second the idea of a second furnace. Around here, lots of houses have two furnaces, especially the larger houses.